Behind the Paper

What happens if a fire breaks out in a road tunnel and visibility suddenly drops to zero? In such a moment, people are forced to make critical decisions under stress, often with very limited information. The distance to the nearest emergency exit can become the difference between safe evacuation and serious harm.

Road tunnels are essential parts of modern infrastructure, yet they also represent one of the most challenging environments in terms of fire safety. Their confined geometry, limited escape options, and the rapid spread of smoke make evacuation particularly complex. While safety regulations exist worldwide, one issue quickly became apparent to us: there is no clear consensus on how far apart emergency exits should be placed. Depending on the country, this distance can vary significantly—from 150 meters to as much as 500 meters—often without a transparent or universally accepted methodology.

This observation led to a simple but important question: Can we define emergency exit spacing based on actual safety conditions rather than fixed, prescriptive values?

From question to methodology

This research began during a postgraduate course, where we had the opportunity to explore real-world fire safety challenges together with a student working on her diploma thesis. With guidance from the supervising professor, we decided to focus on tunnel evacuation during fire incidents—an area where engineering, human behavior, and safety regulations intersect.

Rather than relying on existing rules, we aimed to develop a performance-based methodology. In other words, instead of assuming a “safe” distance, we wanted to calculate it based on what people actually experience during a fire.

To do this, we combined two advanced simulation tools:

• A fire simulation model to represent how heat and smoke spread inside a tunnel
• An evacuation model to simulate how people move and react under those conditions

We evaluated safety using key factors such as:

• Temperature, which can quickly become life-threatening
• Toxicity of smoke, which affects a person’s ability to breathe and function
• Visibility, which directly influences how fast and effectively people can move

A key concept in our work was determining when conditions become untenable—that is, when they exceed what a person can safely tolerate.

What we discovered

One of the most striking findings was that visibility plays a dominant role in evacuation safety. As smoke spreads through the tunnel, visibility rapidly decreases, causing people to slow down significantly. This reduction in movement speed leads to longer evacuation times and increased exposure to hazardous conditions.

Ventilation also proved to be a critical factor—but not always in the way one might expect. While ventilation is generally intended to improve conditions, increased airflow can actually accelerate the spread of smoke throughout the tunnel. In many cases, this resulted in smoke filling the entire cross-section more quickly, reducing the time available for safe evacuation and shortening the maximum distance a person can travel.

Another important factor is reaction time. The longer people delay before starting to evacuate, the worse the conditions become. As the fire develops, temperatures rise and toxic gases accumulate, significantly reducing the safe distance that can be covered.

Interestingly, we also observed specific cases where moderate airflow created slightly improved conditions for people located close to the fire, due to temporary layers of cleaner air. However, this effect was limited and did not change the overall conclusion: smoke propagation is the main threat to safe evacuation.

From simulations to practical insights

Based on our results, we developed a structured approach—a kind of practical framework—that can be used to:

• Estimate the maximum safe distance between emergency exits
• Evaluate the impact of ventilation conditions
• Highlight the importance of fast and effective alarm systems
• Support decision-making in tunnel design and safety planning

This approach allows engineers and authorities to move beyond fixed numbers and instead base their decisions on measurable safety criteria.

To better understand the implications of our findings, we compared our calculated safe distances with existing international regulations. The differences were significant.

Why this matters

Ultimately, this research highlights a key idea: safety should not rely on arbitrary numbers. Instead, it should be grounded in a clear understanding of how people and hazards interact in real conditions.

By adopting a performance-based methodology, it becomes possible to design safer tunnels, optimize emergency exit placement, and support more consistent and scientifically justified regulations across different regions.

In environments where seconds matter and visibility can disappear in moments, designing for realistic conditions is not just an engineering challenge—it is a necessity.

https://www.doi.org/10.1007/s10694-026-01900-8